1. Technical Field
The present invention relates to a phase measurement apparatus, a skew measurement apparatus, a phase measurement method and a skew measurement method. More particularly, the present invention relates to a phase measurement apparatus which samples a signal under measurement at predetermined periods, and uses the result of the sampling to calculate the phase of the signal under measurement.
2. Related Art
A skew between a plurality of signals may be statistically measured by using a device such as a time interval analyzer and a frequency counter. For example, a method is known which uses a time interval analyzer to measure a difference in timing between the zero-crossing points of the signals under measurement, and measure a skew based on the distribution of the measured difference in timing, as disclosed in “Jitter Analysis Clock Solutions”, Wavecrest Corp., 1998, for example.
Another method is proposed which calculates instantaneous phases of the signals under measurement based on the analytic signals of the signals under measurement, and calculates a skew based on a differences in initial phase between the instantaneous phases as disclosed in U.S. Pat. No. 7,127,018.
However, the method performed with the use of a device such as a time interval analyzer has the following problem. After the timing of a particular zero-crossing point is measured, a dead time during which no measurement can be performed is present before the timing of a zero-crossing point is subsequently measured. For this reason, a long time period may be required to gather a sufficient number of samples of data to achieve desirable measurement accuracy.
On the other hand, the method based on the analytic signals also has a drawback. The analytic signals are calculated based on the result of sampling the signals under measurement. Here, to accurately calculate the analytic signals, it is preferable to use an A/D converter that has a resolution of approximately 8 bits in the amplitude axis or to set the sampling period at a sufficiently small value. To detect the logical values of the signals under measurement, however, the measuring pins of a semiconductor test apparatus (automatic test equipment, ATE) or the like use a 1-bit comparator to sample the signals under measurement at the rate of up to 6.5 GHz. This configuration makes it difficult to measure a skew highly accurately with a resolution of approximately 1 ps.